First New Species of North American Medicinal Leech in 40 Years Discovered

Scientists have discovered a new species of medicinal leech lurking in the freshwater wetlands of southern Maryland.

This is the first new identification of a North American medicinal leech since 1975, according to a study published in the Journal of Parasitology—which was led by Anna Phillips from the Smithsonian Museum of Natural History.

"We found a new species of medicinal leech less than 50 miles from the National Museum of Natural History—one of the world's largest libraries of biodiversity," Phillips said in a statement. "A discovery like this makes clear just how much diversity is out there remaining to be discovered and documented, even right under scientists' noses."

Leeches are parasitic worms that feed on the blood of other animals. Those that have a taste for human blood are described as "medicinal leeches."

"'Medicinal leech' is a common name for some leeches that feed easily on humans and could be used for medicinal purposes," Phillips told Newsweek. "There are several species that are considered 'medicinal,' including Hirudo medicinalis, the European medicinal leech, and the Macrobdella species, the North American medicinal leeches."

These creatures have been used since ancient times by physicians for their perceived ability to treat a host of ailments. In fact, they are still used in modern medicine, particularly in situations where a body part—such as a finger—is re-attached after being cut off.

Phillips discovered the new species during an expedition in 2015 during which she collected several olive-green leeches with orange spots in a wetland near the Potomac River, Maryland, less than 50 miles from downtown Washington, D.C.

Initially, she thought that these creatures represented a species that was already known—called Macrobdella decora—and commonly found in many parts of North America.

However, DNA analysis and closer physical examination of the specimens revealed that the animals belonged to a new species, dubbed Macrobdella mimicus. In the new species, the reproductive pores, which are found on the bottom of leech bodies, were positioned slightly differently to M. decora.

"Superficially, this species looks similar to the more broadly distributed and well-known species, Macrobdella decora," Phillips said. "Both species have the same number of accessory pores (which exude mucus during copulation,) but the new species has a different number of annuli (external rings on the body) between the gonopores (reproductive openings) and the accessory pores."

In a subsequent field expedition, Phillips and her team found more leeches with the same pore positioning.

"Then we sequenced [their DNA], and they all came out more closely related to the leeches we had found in Maryland than to anything else known to science," Phillips said.

Noticing a pattern, she then looked at leech specimens kept in parasite collections at various institutions in order to identify others that featured the same pore positioning. She soon discovered that these leeches had been found across a huge swathe of the eastern U.S., dating all the way back to 1937.

"All of a sudden, I started finding these things everywhere," she said. "It's been here this whole time. We just hadn't looked at it in this new way."

medicinal leech
The newly described medicinal leech. Anna Phillips, Smithsonian